Vernonia is one of the leafy vegetables that can be used in an attempt
to alleviate the problem of micronutrient malnutrition, prominent in tropical
Africa. Commonly known as Ndole or Bitterleaf in most Central and West African
countries, it is one of the most widely consumed leafy vegetable in Cameroon.
Though eaten by a large proportion of the population, cultivation is limited
to the southern parts of the country and mostly in the rainy season. In order
to ensure availability in nongrowing areas or seasons, the vegetable is usually
processed and preserved. Processing and preservation methods used are thought
to influence the nutrient content of these vegetables. The study was aimed
at determining the effects of processing and preservation methods on vitamin
C and total carotenoid levels of some species of Vernonia (V. amygdalina,
V. calvoana var. bitter, V. colorata and V. calvoana var.
non bitter) consumed in Cameroon. The processing methods were squeeze-washing,
boiling and squeeze-washing with 0N, 1N and 2N concentrations of natron.
The methods of preservation were sun drying, oven drying at different temperatures
(45, 60 and 75°C) and freezing for 0, 10, 30 60 and 120 days. Results
show that these leafy vegetables were good sources of vitamin C and total
carotenoid. Vitamin C values vary from 137.5±3.3 in V. calvoana non
bitter to 197.5 ± 3.5mg/100g in V. colorata, while total carotenoid
levels range from 30.0 ± 1.0 in V. amygdalina to 41.5±0.9mg/100g
in V. colorata for the raw samples. Greater losses of both vitamins
were observed for the bitter species. Drying caused significant losses, especially
in Vitamin C. The best temperature for drying the vegetables to preserve
carotenoids and vitamin C was at 45°C, whereas sun drying and oven drying
at 75°C caused the highest losses. Freezing for up to ten days had no
significant influence on total carotenoids and vitamin C levels. Comparatively,
squeezewashing proved to be the best processing method that ensured minimum
loss of both vitamins.

Malnutrition persists in developing countries in spite of the increase in production
of basic foods. One of the most common forms of malnutrition is Vitamin
A deficiency which affects about 250 million preschool children world wide
[1]. In Africa, vitamin A deficiency affects close to 18 million preschool
children, of which 1.3 million are affected by xerophthalmia. Problems
related to vitamin C deficiency may not be directly emphasized but its
role in enhancing absorption of iron is to be mentioned, as iron deficiency
is prominent in most developing countries and could very much be linked
to its absorption.

Leafy vegetables are the most available and cheapest source of substantial amounts
of vitamins A and C to the most vulnerable groups [2]. However, in Tropical
Africa, millions of people still suffer from vitamin A and C deficiency
despite the increased consumption of leafy vegetables. These leafy vegetables
are relatively inexpensive, easily and quickly cooked and rich in several
nutrients especially β-carotene and vitamin C, which are essential
for human health. These vegetables also provide some minerals such as iron. Vernonia,
commonly known as Bitterleaf, is widely used in most West and Central African
countries both for human and animal consumption. Though consumed by a large
proportion of the Cameroonian population, cultivation is limited to the
southern parts of the country and in the rainy season. Post-harvest losses
are evident during the six months of Vernonia glut. Therefore there
is need for storage use during the six months of dry season. Two main types
exist in Cameroon; the non-bitter types (V. colorata, V. calvoana var.
non bitter) and the bitter types (V. amygdalina, V. calvoana var.
bitter). Local processing involves squeeze-washing the raw or boiled leafy
vegetable to remove the bitter taste and foam. The leaves are tenderised
and the greenish colour preserved by boiling with natron. The washed bitterleaf
can be preserved by freezing or drying. These processes generally lead
to losses of some nutrients and anti- nutrients (anti nutritional factors).
Processing and preparation of foods brings about losses in nutrients and
the extent of these losses depends on the type of technique used [3].

Studies on the nutritional composition of Vernonia are numerous and limited
to one species: V. amygdalina. In 1990, Faboya demonstrated that
ascorbic acid decreases with storage time [4]. Oshodi found that the dried
leaves of V. amygdalina were rich in minerals, especially in phosphorus,
and that the content of ascorbic acid was temperature dependent [5]. Little
is known about the effect of processing and preservation on the different
species of Vernonia (V. amygdalina, V. calvoana var.
bitter, V. colorata and V. calvoana var. non bitter). This
study was aimed at determining the effect of different processing and preservation
methods on the vitamin A and C levels of four species of Vernonia available
in Cameroon. This information will be useful in determining the best methods
of preservation and processing which cause minimum losses of total carotenoids
and vitamin C in different species of Vernonia.

Materials and Methods

Samples were collected in an experimental farmland in Ngaoundéré,
Cameroon in the months of October and November. The leaves and growing
shoots were sorted, rinsed under running tap water, left to drain, and
subsequently sliced using a stainless steel kitchen knife before submitting
them to the different treatments.

To determine the effect of squeeze-washing and boiling with or without natron,
a 5x4 factorial design was used. The species V. amygdalina, V. calvoana var.
bitter, V. colorata and V. calvoana var. non bitter were
subjected to five different treatments: Raw (R), squeeze-washing (W), squeeze-washing
and boiling in 0N (Wb), 1N (W1) and 2N (W2). Concentrations of natron (0N)
indicates boiling without natron, whereas 1N is the average quantity of
natron used by housewives - 10 g/ 450 g fresh leafy vegetables boiled to
tenderise the leaves and preserve the colour for ten minutes.

To determine the influence of drying, a 5x2x4 factorial design was used. Five
treatments as above, two species (V. amygdalina and V. calvoana var
bitter) and four conditions of drying- sun drying and drying in a moisture
oven at 45 °C, 60 °C and 75 °C.

The effect of duration of freezing was determined on only one species (V. calvoana),
stored at -18°C in a freezer for 10, 30, 75, and 120 days (5x5 factorial
design). All samples were later dried, ground to fine powder and stored
in a refrigerator inside air tight containers awaiting analysis

The vitamin C levels were determined using N-Bromo-succinimide recommended for
the pigmented solutions, while total carotenoid was first extracted using
a mixture of hexane - acetone 30/70 (v/v) then separated by column chromatography
and quantified using a spectrophotometer [6, 7]. All the analyses were
done using triplicate samples. Experimental results were subjected to Analysis
of Variance (ANOVA) and differences between means were assessed by Duncan's
new multiple range test using the statistical package statsgraphics 2000.

RESULTS

Changes in vitamin C levels

Table 1 shows the levels of vitamin C in the four species of Vernonia as
affected by different processing techniques. Vitamin C values for the raw
leaves of all four species of Vernonia varied from 137.5 ± 3.3
in V. calvoana var. non bitter to 197.5 ± 3.5 mg/100 g in V. colorata.
These values were generally high for all the raw species of Vernonia as
compared to the processed samples. However, losses were noticed between
55% and 77% in V. calvoana var. bitter and V. amygdalina respectively,
with the greatest losses in the bitter species (V. amygdalina and
V. calvoana var. bitter). For processing conditions, simple squeeze-washing
had the least loss of vitamin C while the highest destruction was observed
when natron was used (P<0.05).

Table 1. Vitamin C levels in raw and processed forms of different species
of Vernonia (mg/100 g)

V. calvoana

V. calvoana

V. amygdalina

var. bitter

V. colorata

var. non bitter

R

166.5±2.1a

178.5±16.2 a

197.5±3.5 a

137.5±3.3 a

W

75.5±6.4 b

117.0±12.7 b

95.0±4.2 b

98.5±16.3 b

Wb

57.5±6.4 c

79.5±2.1 c

90.0±4.2 b

95.0±7.1 b

W1

66.0±1.4 bc

97.0±18.4 b

77.0±1.4 c

53.0±1.8 c

W2

38.0±1.4 d

63.0±2.8 c

75.0±2.1 c

51.0±4.2 c

Values are means±SD Means not sharing a common superscript letter in a
column are significantly different at p< 0.05 R =Raw, W= Squeeze-washing,
Wb = Boiling and squeeze-washing. W1 boiling in1N natron and squeeze-washing;
W2= Boiling in 2N natron and squeeze-washing

The effect of temperature on the vitamin C levels of V. calvoana var.
bitter and V. amygdalina is presented in Tables 2 and 3, respectively.
Vitamin C levels dropped with increase in temperature. The highest losses
were found in the sun-dried samples for either the raw or processed samples.
Raw samples of V. amygdalina had values that dropped from 166.5 ± 2.1
mg/100 g to 73.0 ± 9.9 mg /100 g, and the values for V. calvoana var
bitter also dropped from 178.5 ± 16.2 to 61.0 ± 12.7 mg /100
g, ie 56% loss for the two species when sun dried. The losses due to drying
in the processed samples were lower for both species than for the raw samples.

Table 4 presents the effect of freezing duration on the vitamin C levels of
V. calvoana var. bitter. For both the raw and processed leafy vegetables,
freezing for between zero and ten days showed no significant difference
(P>0.05) in the values of vitamin C as was in the case when these samples
were frozen for 30, 75 and 120 days.

Changes in total carotenoids

The levels of total carotenoids in the different species subjected to different
processing techniques are shown in Table 5. Total carotenoids levels range
from 30.0 ± 1.0 in V. amygdalina to 41.5 ± 0.9 mg /100
g in V. colorata for the raw samples. These raw samples were considerably
reduced when subjected to different processing methods (P<0.05). More
losses of carotenoids were observed for the bitter species (V. amygdalina had
50% and V. calvoana var. bitter had 67%). Meanwhile V. calvoana var.
non bitter had 43% and V. colorata had 49% loss. The greatest losses
were noticed when 2N natron was used in processing. Table 6 and 7 show
the effect of different drying conditions on the carotenoid levels of V. calvoana and amygdalina respectively.
Losses due to drying were significant for the raw samples with maximum
losses of up to 43% when samples of V. amygdalina were dried at
75 °C, while carotenoid levels dropped from 38.5 ± 0.3 to 26.1 ± 0.2
mg /100 g, ie 32% when solar radiation was used for drying V. calvoana var.
bitter. Table 8 shows the effect of duration of freezing at -18°C
on the carotenoid levels of Vernoniacalvoana. Significant
losses were observed after ten days of freezing the raw samples at P< 0.05.
Freezing for ten days resulted to significant variation for both the raw
and processed samples of V. calvoana var. bitter (P>0.05)

Table 5. Levels of carotenoid of the different species of Vernonia (mg/100
g) after processing

V. calvoana

V. calvoana

V. amygdalina

var .bitter

V. colorata

var. non bitters

R

30.0±1.0 a

38.5±0.3 a

41.5±0.9 a

35.8±0.2 a

W

21.6±0.4 b

16.9±1.0 b

26.0±0.5 b

28.5±0.2 b

Wb

21.9±0.4 b

16.5±1.4 b

21.2±3.3 b

27.8±1.8 b

W1

16.4±3.4 c

14.9±1.8 b

22.5±1.0 b

21.8 ± 3.9 c

W2

14.9±0.4 c d

12.8±0.4 c

21.2±2.1 b

20.2±0.3 c

Values are means±SD Means not sharing a common superscript letter in a
column are significantly different at p< 0.05 R =Raw, W= Squeeze-washing,
Wb = Boiling and squeeze-washing. W1 boiling in1N natron and squeeze-washing;
W2= Boiling in 2N natron and squeeze-washing

Vitamin C values were high for all four species of Vernonia with as much
as 77% losses after processing. Singh et al. obtained closely similar values
for vitamin C in some Indian leafy vegetables [8]. Bender also observed
similar trends, and proved that in leafy vegetables, losses in vitamin
C were a function of the method of processing [3]. In this same light Machlin,
found 50% loss after cooking V. amygdalina [9]. These losses are
justified since vitamin C is thermo sensitive and hydrosoluble. Losses
were also found in samples that were dried at different conditions. The
maximum losses observed after drying samples at 75°C and using solar
radiation, was confirmed by the works of Oshodi who proved that vitamin
C levels in vegetables are temperature dependent [5]. Generally vitamin
C levels in V. calvoana var. bitter were stable when these vegetables,
processed or raw were stored at -18°C for ten days. After this period,
significant losses were observed. Faboya observed that vitamin C could
be conserved by storing leafy vegetables in a refrigerator [3]. The losses
observed after ten days of freezing were more likely to be during the thawing
process when some water is lost together with some Vitamin C [9].

Vitamin A deficiency remains a major problem in Cameroon, affecting mostly the
people in the Northern provinces [10, 11]. Its role in vision and growth
regulation has made the public health officials to look for urgent and
rapid methods of combating the problem. Leafy vegetables remained one of
the most important and cheapest sources of Vitamin A. Processing generally
leads to losses [12]. High levels of carotenoids, found in the raw samples
of the different species of Vernonia were destroyed during processing.
Higher values of carotenoids were found in fresh bengal gram leaves [8]
but values fall within the range of values obtained for spinach and amaranth
[13, 14]. Processing caused losses and these losses were more in the bitter
species: V. amygdalina and V. calvoana var. bitter. This
could be explained by the fact that processing was more intense in the
bitter species. High losses, also found when 2N natron was used in processing
proved that though carotenoid is not hydrosoluble, it could be destroyed
by the use of alkaline during cooking. The losses observed as a result
of processing in the present study are higher than those in the literature
[12, 15], and this can be attributed to variations in processing methods.
Losses in total carotenoids were observed when the different species of Vernonia were
dried under different conditions. Yamini, however, found varying trends
due to drying of different leafy vegetables in India at different conditions
[16].

CONCLUSION

The different species of Vernonia are good sources of
Vitamin A and C. However, processing generally leads to significant losses.
Squeeze-washing and rinsing seem to be the best treatment to preserve
both vitamins A and C. Losses were more prominent in the bitter species
that require intense squeeze-washing and rinsing. Drying at 45°C
is best recommended while freezing for less than ten days is most appropriate
for the conservation of vitamin A and C. Nevertheless, because of the
increased demand for these nutrients, and limited possibility to avoid
processing and conservation, it is pertinent that supplementation of
Vitamin A and exploitation of other sources of Vitamin C - such as from
fruits - be considered imperative.

Acknowledgement We thank the International Foundation for Science (IFS) Sweden, and the Organisation of Islamic Conference, Pakistan for their financial support.